Screw compressor with cooling

ABSTRACT

The invention provides a two-rotors screw compressor having active cooling means on the pressure side of the motive rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the national stage of International Application No.PCT/EP96/02631 filed Jun. 18, 1996.

BACKGROUND OF THE INVENTION

In screw-spindle compressors, as disclosed by EP-A 472933, the pressuredifference attainable depends to a considerable extent on the leakagelosses between the peripheral surfaces, moving relative to one another,of the rotors and the pump-chamber housing. In view of this, the aimwill be to keep the clearance between these surfaces as small aspossible. However, the operating safety, with due regard to thetemperature-induced thermal expansion of the rotors, requires greaterclearance.

It is known to directly cool rotors of twin-shaft compressors (EP-A290664) by a heat-transfer medium (lubricating oil) being provided in abearing hollow space of the rotor, which heat-transfer medium is cooledby a stationary cooling coil projecting into the bearing hollow space.This has the disadvantage that the bearing hollow space of the rotor hasto be sealed off. However, the seals required for this aretrouble-prone, in particular at a high number of revolutions. Highlosses which lead to the generation of heat and jeopardize the coolingeffect also arise in the heat-transfer medium, which is swirled betweenthe rotating rotor and the stationary cooling coil.

It is conventional practice to cool the delivered medium by liquidcoolant, for example, being injected (U.S. Pat. No. 4,515,540) or bysome of the delivered medium being fed back after cooling (DE-A 25 44082). Such cooling may also be provided in combination with theinvention; however, the aim of the invention is to cool the rotor sothat the rotor, in particular in the area of the sensitive bearings, canassume a temperature which is below the pressure-side temperature of thedelivered medium.

SUMMARY OF THE INVENTION

The object of the invention is therefore to create a screw-spindlecompressor of the type described, in which the rotors are cooledindependently of the delivered medium in such a way that goodpreconditions for a small clearance between the rotors themselves aswell as between the rotors and the pump-chamber housing are createdwithout requiring trouble-prone seals.

The solution according to the invention is composed of two components,namely firstly the feature that the displacement rotors are cooled to agreater extent on the pressure side than on the suction side andsecondly a cooling technique utilizing the special type of constructionof the rotor bearing arrangement.

The idea of cooling the rotors to a greater extent on the pressure sidethan on the suction side is based on the fact that, in these machines,most of the compression heat arises in the pockets located closer to thepressure side and enclosed by the rotors and the pump-chamber housing,since, as a result of the leakage losses and possibly also thepreadmission at possibly the same volume, they contain a greater gasmass than the pockets closer to the suction side. If the heat ispreferably dissipated from the rotor area close to the pressure side,constant diameter ratios of the rotors over their entire length will beachieved more easily than if the rotors are cooled over their entirelength. Here, multi-stage rotors mean those whose screw turns formingthe compression pockets orbit the rotor several times, so that aplurality of compression pockets separated from one another in each caseon the suction and pressure side are formed over the rotor length. In athree-stage arrangement, the screw turns orbit the associated rotorthree times in each case. The stage number may be established inaccordance with the respective range of pressure application. At leastfive stages are preferably used.

For the cooling, the invention uses a special technique adapted to thetype of construction. This type of construction requires eachdisplacement rotor to be mounted in a floating manner on a stationarybearing tube surrounding the rotor shaft and at least one rotor-sidebearing and projecting into the rotor. Only the bearing tube is directlycooled, while the cooling of the rotor takes place indirectly by theperipheral surfaces, opposite one another, of the rotor and the bearingbody being arranged in such a way as to be capable of heat exchangerelative to one another. The bearings and the rotor shaft are cooledespecially effectively, since they are located inside the bearing tube.

In order to improve the heat transfer between the surfaces, opposite oneanother, of the rotor and the bearing body, these surfaces may beprovided with properties improving the heat exchange. So that theconvective heat exchange by means of the air layer located between thesurfaces is intensified, the intermediate space should not be connectedto the suction side but to the pressure side. The surfaces may also beprovided with prominences and depressions which improve the coefficientof heat transfer to the medium located in between. The distance betweenthe two surfaces should be as small as possible. To improve theradiation exchange, such a treatment of the surfaces can be providedthat they have a high absorption factor in the area of the heatradiation.

The heat transfer to the surfaces, opposite one another, of the rotorand the bearing body can also be improved by the gas located in betweenbeing set in flow motion. For this purpose, the intermediate space canbe connected to a gas source. The gas flow may also be used for the heatdissipation if an appropriately low gas temperature (if need be cooling)is selected. In addition, it may possibly perform a sealing function forprotecting the bearing and drive area from the admission of the deliverymedium or from substances contained in the delivery medium.

The used gas is expediently fed to the pressure side of the machine. Todeliver the gas, the interacting surfaces of rotor and bearing body maybe equipped with delivery members. It may consequently be unnecessary toprovide an external compressed-gas source. This also applies when thefed gas is primarily intended to be used not for cooling purposes butfor sealing purposes. The delivery action of the surfaces can be broughtabout in particular owing to the fact that they are equipped withdelivery threads on one side or both sides. Instead or in addition, theymay also be of conical design so that the action of the centrifugalforce is utilized for the delivery. Such means encouraging the motion ofthe gas in the intermediate space are also useful for improving the heattransfer when no additional gas feed is provided.

The part of the bearing body projecting into the rotor hollow space isexpediently equipped with passages through which cooling fluid flows andwhich are preferably arranged close to the peripheral surface of thebearing body opposite the rotor.

Since the thermal expansion of the rotor is limited thanks to thecooling according to the invention, the housing may be cooledintensively or at least kept at a predetermined temperature without therisk of the rotor running against the housing due to thermal absorptionof the clearance. The efficiency of the pump can be increased by thecooling action exerted in this way on the delivery medium.

It is known in particular in the case of vacuum pumps to allow a gasunder higher pressure to flow into the compression cells of the machinein order to cool the delivery medium and/or reduce noise. This techniquedesignated as preadmission is also used with advantage in connectionwith the invention. For example, cooled gas from a suitable source maybe used. An external heat exchanger can be avoided by passing thepreadmission gas through a heat exchanger located in the cooling pocketon the housing side. Instead of gas, liquid may also be fed into thepump chamber, which liquid vaporizes there and thereby extracts heatfrom the delivery medium.

The cooling of the bearing body at least in that area in which thebearing body is affected by the heat of the rotor has the greatadvantage that rolling bearings may be used which are permanentlylubricated with grease and therefore require especially littlemaintenance and constitute no contamination hazard for the pump chamber.

The abovementioned possibility of equipping the interacting surfaces ofrotor and bearing body with delivery members can be utilized to protectthe bearing area from foreign substances which could come from the pumpchamber. For this purpose, the interacting delivery members are designedwith a delivery direction leading out of the rotor hollow space.

Foreign substances, especially substances specifically heavier than thedelivery medium, during the feeding of sealing medium also the deliverymedium itself, are thereby prevented from penetrating into the rotorhollow space against the delivery direction and from advancing into thebearing and drive area. This action is assisted by the force of gravity.

In an advantageous embodiment, the interacting surfaces are designed asdelivery members by at least one of them being provided with a deliverythread. It is also possible for both to be provided with deliverythreads. The direction of the thread or threads is selected in such away that the desired delivery direction results. In another embodimentof the invention, the peripheral surfaces, opposite one another, of therotor and the bearing body run conically with a diameter increasing inthe delivery direction, so that the centrifugal force drives backpenetrating substances for instance in the direction of the increasingdiameter, that is towards the pump chamber. A plurality of such deliverymeans (e.g. delivery threads and conicity) may also be combined with oneanother.

This action is increased by connecting the rotor hollow space to aflushing- or sealing-gas source. Thanks to the delivery action, thissource need not be under positive pressure; however, this is not out ofthe question. The gas may also be used for cooling purposes.

An especially important consequence of the invention is the safetyagainst the ingress of liquid into the bearing and drive area.Consequently, the pump not only becomes insensitive to liquid surge withregard to the sealing action but it can also be specifically flushed, inparticular for cleaning. For this purpose, special devices may beprovided for the admission of a washing liquid, which serves, forexample, to release and flush out impurities deposited on the rotor orhousing surfaces. If in the meantime the rotational operating speedcannot be maintained, the rotors should be driven at an appropriatelyreduced speed. Appropriate control devices can be provided for this. Itis especially simple and advantageous to control the rotational speed asa function of torque, since the reduction in the rotational speed thenoccurs automatically. The reduction in the rotational speed can beslight if relatively small quantities of liquid are merely injected intothe gas-delivery flow. The greater the proportion of liquid in thefilling of the delivery spaces, the lower will be the rotational speedwhen driving as a function of torque. Complete flooding of the pumpchamber may even be provided as long as the low rotational speed thenpossible and the delivery action still present here in the intermediatespace between rotor and bearing body are sufficient in combination withthe geodetic height of the bearing body inside the rotor to prevent theoverflow of the flushing liquid into the bearing area.

Safety against the passage of liquid in both the operating state and thestate of rest can be achieved by the invention. The force of gravity andthe pressure difference act in both states and the delivery membersadditionally act in the operating state.

The invention is explained in more detail below with reference to thedrawing, which illustrates a longitudinal section through anadvantageous exemplary embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Resting on the foot part 1 is the motor housing 2, which is connected,if need be in one piece, at the top to the flange-like base plate 3 onwhich the pump-chamber housing 4 is mounted. The latter is closed off atthe top by a lid 5 which contains a suction opening 6.

Fastened to the base plate 3 in a manner to be explained later are theflange plates 50 of the bearing bodies 7, which in each case serve tocarry a rotor 8, the periphery of which has displacement projections 9which are preferably arranged as a two-start helix and engage like themeshing of teeth in the delivery hollow spaces 10 between thedisplacement projections 9 of the adjacent rotor. In addition, thedisplacement projections 9 interact at the periphery with the innersurface of the pump-chamber housing part 4. The rotors 8 are connectedat the top to the suction space 11 and at the bottom to the pressurespace 12.

The pressure space 12 is connected to a pressure outlet (not shown).These parts are provided at the bottom end of the vertically mountedpump-chamber housing.

Each rotor 8 is connected in a rotationally locked manner to a shaft 20which is mounted at the bottom in the bearing body 7 by a permanentlylubricated rolling bearing 21. A second, likewise permanently lubricatedrolling bearing 22 is located at the top end of a tubular part 23 of thebearing body 7, which projects into a concentric bore 24 of the rotor 8,which bore 24 is open towards the bottom, i.e. on the pressure side.This bearing 22 is preferably located above the centre of the rotor 8.The tubular part 23 of the bearing body preferably extends through mostof the length of the rotor 8. In a vertical arrangement of the pump, theend of the tubular part 23 lies substantially higher than the pressureoutlet 17. This helps to protect the bearing and drive region from theingress of liquid or other heavy impurities from the pump chamber.

Provided in the tubular part 23 of the bearing body are cooling passages25 which are connected via passages 26 to a cooling-water source and viacorresponding passages (not shown in the drawing) to a cooling-waterdischarge. The cooling passages 25 are preferably formed by helicalturned recesses which are tightly covered by a sleeve. The cooling ofthe rotor bearings prolongs the service life or the maintenanceintervals of these bearings if they are permanently lubricated withgrease. Furthermore, the peripheral surface of the tubular part 23 ofthe bearing body is also kept at a low temperature by the cooling. Thisperipheral surface is opposite the inner peripheral surface of thehollow space 24 of the rotor at a slight distance apart. These surfacesare designed in such a way that they are capable of good heat exchangeand therefore heat can be dissipated from the rotor indirectly via thetubular part 23 of the bearing body and its cooling devices 25. Thesurfaces, opposite one another, of the tubular part 23 of the bearingbody and the rotor hollow space 24 may be designed in a suitable mannerin order to improve the heat exchange between them. For example, theymay be treated or burnished in such a way that the radiation exchange ispromoted by high absorption coefficients. The convective heat exchangeby means of the gas layer in between may be improved by a small surfacespacing and a suitable surface structure which leads to the increase inthe coefficient of heat transmission. For this purpose, one surface orboth surfaces may be designed with a coarse finish or with heat-exchangeribs or threads or the like. It is also possible to feed a sealing gasto the rotor hollow space 24 through the bearing body or the shaft 20,which sealing gas is discharged with the delivery medium from thepressure space 12. Apart from the sealing of the bearing region, it canalso serve to additionally cool the bearing, the bearing body and therotor, but in this case it is expediently not directed through thebearing or bearings in order not to contaminate the latter but isdirected via a passage 28 forming a bypass.

To protect the bearing and drive area from inflows penetrating from thepump chamber, suitable sealing and/or barrier devices are provided. Itis especially advantageous to equip the opposite surfaces of the bearingbody 23 and the inner surfaces of the rotor hollow space 24 with adelivery thread (not shown) on one side or both sides, which deliverythread exerts a delivery effect from the rotor hollow space 24 towardsthe pressure space 12. This delivery effect mainly acts on solid orliquid particles on account of their higher density and thereby preventstheir ingress into the bearing and drive area. The delivery thread isexpediently designed in such a way that this effect is still active evenat a considerably reduced rotational speed.

The delivery effect can also be brought about by the gap between rotorand bearing body widening conically towards the pressure space. Here,the gap width (distance of the surface of the bearing body from thesurface of the rotor) remains essentially constant. In addition, thesurfaces opposite one another may also be provided in this case with adelivery thread on one side or both sides, but this is not necessary.

Since the equipping of the gap between rotor and bearing body with adelivery thread or conicity acting in a delivering manner provides avery effective seal against the ingress of liquid or solid particles,additional sealing devices may often be dispensed with; however, theymay be provided, and in fact preferably in a non-contact orminimum-contact type of construction, e.g. labyrinth seals orpiston-ring-like seals.

On account of the sealing action of the delivery thread or the gapconicity, the pump according to the invention is insensitive to thepresence of liquid in the pump chamber as long as the rotors arerotating. This insensitivity also exists in the stationary state owingto the high bearing arrangement in the rotor as long as the liquid inthe pump chamber does not reach the bearing level. It is not onlyimportant when the delivery medium carries a liquid surge with it butmay also be utilized for cleaning and/or cooling the pump by liquidinjection. For example, cleaning or cooling liquid can be injectedthrough nozzles, of which one is indicated at 27. The same or separatenozzles 27 may be used for injecting the cleaning liquid and the coolingliquid.

If very severe contamination has to be expected, it is possible toconstantly inject cleaning liquid during operation. During the operationof a vacuum pump, the cleaning liquid, provided it can pass into thepump chamber, should have a vapour pressure below the intake pressure.If the pump is a multi-stage pump and the contamination (for example asa function of pressure) settles mainly in the second and/or followingstages, it is possible to limit the injection of the cleaning liquid tothe second or following stage and to thereby separate it from thesuction side.

In most cases, however, the cleaning operation does not take placeconstantly but periodically if a requirement for cleaning (for exampleas a result of an increase in the drive torque) is established. Owing tothe insensitivity of the pump to liquids, relatively large liquidquantities may then also be used. If the rotational operating speedcannot be maintained on account of the quantity or type of cleaningliquid used, the rotational speed may be reduced accordingly. Suitablecontrol devices are provided for this. For example, the rotational speedmay be controlled as a function of the drive torque, which automaticallyleads to a corresponding reduction in the rotational speed relative tothe rotational operating speed at increased power requirement. Thecontinuous rotation of the rotors even during the cleaning phase notonly serves to seal the rotor bearing arrangement but also conveys theeffect of the cleaning liquid to the contaminated surfaces.

The delivery action in the gap between rotor and bearing body may alsobe utilized to deliver sealing gas independently of an externalcompressed-gas source. However, to deliver the sealing gas, the actionof such a compressed-gas source will generally be preferred in order tofeed the sealing gas independently of the rotor speed. Cooling of thehousing shell is not necessary in all cases. However, in the contextaccording to the invention it is advantageously possible, since therotors 8 are also cooled and their thermal expansion is thereforelimited. It need not be feared that the rotors run against the housingonly because they expand, while the housing is kept at a lowertemperature.

The pump according to the invention may be provided with preadmission.This means that passages 31 are provided in the areas of higher, orpossibly even average, compression in the housing, through whichpassages 31 gas of higher pressure than corresponds to the compressionstate in this area of the pump chamber is let into the pump chamber inorder to effect cooling and/or noise reduction according to knownprinciples. According to an advantageous feature of the invention, thepreadmission gas can be extracted directly from the pressure side of thepump by being cooled.

The rolling bearings 21, 22 in the example shown are angular-contactball bearings which are set against one another by a spring 29. Eachshaft 20 carries the armature 35 of the drive motor below the bearing21, preferably directly, i.e. without an intermediate coupling, thestator 36 of which drive motor is arranged in the motor housing 2. Themotor housing may be provided with cooling passages 38.

The flange plates 50, which in the example shown are made in one piecewith the bearing bodies 7, are mounted with their outer margins 51,which essentially follow the periphery of the pump-chamber housing 4,and their abutting inner margins 52 on the top side of the base plate 3.The flange plates 50 are sealed relative to the base plate 3. The endfaces 53, which follow a secant in radial section and at which theflange plates 50 bear against one another, are also provided with asealing insert.

A turned recess is provided below the flange plates 50 between themargins 51, 52, which turned recess encloses with the top side of thebase plate 3 a space 39 which serves to accommodate synchronization gearwheels 40 which are arranged in a rotationally locked manner with knownmeans on the shafts 20 between the bearings 21 and the motor armatures.So that they can mesh with one another in the area of the inner margins52 of the flange plates 50, the inner margins have a cut-out at anappropriate point, through which cut-out the gear wheels reach.Remaining below this cut-out on each side is a web to which thereference line of the reference numeral 52 generally designating theinner margin points in FIG. 1. This web is advantageous not only forstability reasons but also because it permits an encircling seal on theone hand relative to the base plate 3 and on the other hand between theflattened secant faces of the flange plates 50.

The turned-out portions 39 in the flange plates 50 have a diameter whichis greater than the diameter of the synchronization gear wheels 40. Theyare arranged with slight eccentricity in relation to the inner margins52 so that the synchronization gear wheels 40 can be inserted uponassembly of the rotor construction units despite the presence of thesealing web at 52.

Since the space 39 containing the synchronization gear wheels 40 iscompletely separate from the pump chamber, there is no risk of thesynchronization gear wheels becoming contaminated. They are merely usedfor the emergency synchronization of the rotors. Their teeth normally donot come in contact with one another. Lubrication is thereforeunnecessary as a rule. Although it may be used if desired, the dryrunning of the synchronization gear wheels simplifies the construction,since sealing between the space 39 and the drive motors is notnecessary.

The synchronization gear wheels 40 may also serve as pulse generatordiscs or may be supplemented by additional pulse generator discs whichare scanned by sensors 42, of which one is shown in FIG. 1. Thesesensors 42 are connected to a control device which monitors therespective rotary position of the rotors relative to a set point andcorrects it via the drive. This concerns electronic synchronization ofthe rotors, which is known as such and therefore need not be explainedin more detail here. The play between the teeth of the synchronizationgear wheels 40 is slightly smaller than the flank clearance between thedisplacement projections 9 of the rotors 8. However, it is greater thanthe synchronization tolerance of the electronic sychronization device.During proper functioning of the latter, therefore, neither the flanksof the displacement bodies 9 nor the teeth of the synchronization gearwheels 40 come in contact with one another. In the event of the latternonetheless coming in contact with one another, they are provided with awear-resistant and if need be slidable coating.

The performance data of the pump, apart from being determined by thedrive output and rotational speed, are determined by the displacement ordelivery volume formed at the rotors and thus by the length of therotors. The delivery data may therefore be altered by altering thelength of the pump part containing the rotors. A series of pumps havingdifferent performance data is therefore preferably distinguished by thefact that the individual pumps of this series differ through graduationof the length of these parts, to which the pump-chamber housing, therotors and if need be the tubular parts, projecting into the rotors, ofthe bearing bodies belong.

It will be recognized that each rotor forms with the associated bearingand drive devices a construction unit which can be mounted independentlyand, apart from the rotor, consists of the bearings 21, 22, the bearingbody 7, the cooling devices provided therein, the shaft 20, thesynchronization gear wheel 40, the associated sensor 42 and the motorarmature 35. These units are inserted into the pump in a completelypreassembled manner. They can easily be removed from the base plate 3 orinserted after removal of the pump-chamber housing. The exchanging ofthese units can therefore be left to the user, whereas the manufacturertakes care of the maintenance of the sensitive units as such.

The pump is preferably of isochoric type of construction so that largerliquid quantities can also be safely delivered.

We claim:
 1. A screw compressor having two screw rotors comprising asuction side, a pressure side and a rotor shaft, a stationary bearingtube mounting a rotor on the pressure side thereof and enclosing therotor shaft, and at least one rotor-side bearing enclosed by the bearingtube, said bearing tube protruding into the rotor and having aperipheral surface in confronting relationship with a complementarysurface of the rotor, said bearing tube being actively cooled by a fluidforced to flow through a cooling passage, said peripheral andcomplementary surfaces being arranged in such a way as to be capable ofheat exchange relative to one another whereby the rotor is cooled to agreater extent on the pressure side than on the suction side by virtueof the fact that the part of the bearing tube protruding into the rotoris cooled by said heat exchange.
 2. Compressor according to claim 1,characterized in that an intermediate space is provided between theperipheral and complementary surfaces of the rotor and the bearing tubeand said space is connected to the pressure side.
 3. Compressoraccording to claim 1, characterized in that at least one of the saidperipheral and complementary surfaces is provided with an irregularsurface having prominences and depressions that increase the heattransfer area, the improving the heat exchange with a medium located inbetween.
 4. Compressor according to claim 1, characterized in that thesaid peripheral and complementary surfaces are provided with a surfacefinish having high absorption factor for heat radiation.
 5. Compressoraccording to claim 1, characterized in that at least the part of thebearing tube projecting into the rotor contains passages through whichcooling liquid flows.
 6. Compressor according to claim 5, characterizedin that the cooling passages are arranged close to the peripheralsurface of the bearing tube opposite the rotor.
 7. Compressor accordingto claim 1, characterized in that a slight clearance is provided betweenthe peripheral and complementary surfaces and said surfaces are designedas delivery members interacting in a non-contacting manner and having adelivery direction leading out of the rotor.
 8. Compressor according toclaim 7, characterized in that it is arranged essentially verticallywith an outlet opening situated in a geodetically low position. 9.Compressor according to claim 7, characterized in that the peripheraland complementary surfaces opposite one another are of conical designwith a diameter increasing in the delivery direction.
 10. Compressoraccording to claim 1, characterized in that a bore of the rotor isprovided and is connected to a sealing-gas source, solid sealing-gasproviding a sealing function by its flow.
 11. Compressor according toclaim 7, characterized in that means are provided for the control of therotor drive as a function of torque.
 12. Compressor according to claim11, characterized in that means are provided for the admission of awashing liquid to the rotor.